Cellular wireless base stations are able to simultaneously support two-way communications with multiple mobile users on multiple channels using a single antenna without interference among the channels. Avoidance of cross-channel interference is possible because these cellular wireless frequency bands are either split into separate bands for transmit and receive—known as frequency division duplex operation, or the same band and channels are used for transmit and receive but the channels are divided into transmit and receive time slots which are synchronized in time so that all channels transmit or receive at the same time—known as time division duplex operation. By designing the cellular system so that mutual interference is avoided, cellular wireless base stations can then use multi-channel transmitters and receivers that share the same antenna to simultaneously transmit and/or receive on multiple channels.
The commercial aviation very high frequency (VHF) communication (COM) band has not been planned in a manner where a single antenna can be used to simultaneously support two-way communications on multiple channels. The aeronautical VHF COM band occupies a range from 118 MHz to 137 MHz. The VHF COM band is divided into channels that for any single two-way COM link, the same channel must be used for transmit or receive. Each channel can be used to communicate with multiple voice users using a push-to-talk discipline or for data link communications where the transmitting station must first sense whether the channel is being used before transmitting a message to another user. Interference among the various users that share the same channel is avoided in this manner. However when simultaneous voice and data communications on different channels must be supported, coordination between channels to avoid interference is not possible. Consequently aircraft are typically equipped with 3 VHF radios and 3 VHF antennas with each radio/antenna pair used to support a single voice or data COM link. The 3 antennas must be separated far enough so that when a transmission is made from one radio/antenna pair, the emissions from that antenna will not block reception on the other nearby radio/antenna pairs.